MOS field-effect-transistor (MOSFET) is short for metal-oxide-semiconductor field-effect-transistor, which is a kind of semiconductor device that controls electrical current in semiconductor using the effect of electric field. It is also called unipolar transistor because it relies on one type of charge carriers to participate in conducting electric current. MOS field-effect-transistors can be built using semiconductor materials such as silicon or germanium, or semiconductor compounds such as gallium arsenide, while silicon is mostly used currently. Typically, a MOS field-effect-transistor is comprised mainly of a semiconductor substrate, source and drain, a gate oxide layer and a gate electrode. Its basic structure is generally a four-terminal device, and its middle part has a MOS capacitor structure comprised of metal-insulator-semiconductor. The source and drain are on two sides of the MOS capacitor. In normal operation mode, charge carriers enter through the source and exit through the drain. Above the insulator layer is the gate electrode. By applying a voltage to the gate electrode, the strength of the electric field in the insulator layer can be changed, and the electric field at the surface of the semiconductor can be controlled, thereby changing the conductivity of a surface channel.
A mixed junction is a mixture of a Schottky junction and a P-N junction. It has the advantages of high operating current, fast switching speed, small leakage current, high breakdown voltage, etc.
An asymmetric source-drain field-effect transistor has asymmetrically structured source and drain, one of which is made of a P-N junction while the other being made of a mixed junction, the mixed junction being a mixture of a Schottky junction and a P-N junction. Such a transistor has relatively low leakage current, and its source-drain serial resistance is smaller than the source-drain serial resistance associated with conventional heavily doped P-N junction source-drain field-effect transistors.